Lambda Multi-Arch Builds: arm64 CI Matrices That Actually Ship

# .github/workflows/lambda-multiarch.yml
strategy:
 matrix:
 arch: [x86_64, arm64]
 include:
 - arch: x86_64
 platform: linux/amd64
 lambda_arch: x86_64
 - arch: arm64
 platform: linux/arm64
 lambda_arch: arm64
jobs:
 build:
 runs-on: ubuntu-latest
 steps:
 - uses: docker/setup-qemu-action@v3
 - uses: docker/setup-buildx-action@v3
 - name: Build inside Lambda Node 20 image
 run: |
 docker run --rm --platform=${{ matrix.platform }} \
 -v "$PWD":/var/task -w /var/task \
 public.ecr.aws/lambda/nodejs:20 \
 bash -lc "npm ci && npm run build && npm prune --omit=dev"
 - name: Assert ELF arch
 run: ./scripts/assert-native-arch.sh ${{ matrix.lambda_arch }}
 - run: (cd dist && zip -r ../fn-${{ matrix.lambda_arch }}.zip .)

# scripts/assert-native-arch.sh
#!/usr/bin/env bash
set -euo pipefail
want="$1" # x86_64 | arm64
while IFS= read -r -d "" f; do
 info=$(file "$f")
 case "$want" in
 arm64) echo "$info" | grep -q "ARM aarch64" ;;
 x86_64) echo "$info" | grep -q "x86-64" ;;
 esac || { echo "arch mismatch: $f ($info)"; exit 1; }
done < <(find node_modules dist -name "*.node" -print0 2>/dev/null)

new lambda.Function(this, "Api", {
 runtime: lambda.Runtime.NODEJS_20_X,
 architecture: lambda.Architecture.ARM_64,
 code: lambda.Code.fromAsset(`artifacts/fn-arm64.zip`),
 handler: "index.handler",
});

// BAD: default arch in one stack, zip built for another
new lambda.Function(this, "Api", {
 runtime: lambda.Runtime.NODEJS_20_X,
 code: lambda.Code.fromAsset("fn.zip"), // mystery arch
 handler: "index.handler",
});